As the production of biofuels capable of replacing petroleum increases in order to solve the problems of depletion of petroleum resources and global warming, the utilization of biomass is increasing gradually. The biofuels used as alternative energy are prepared by fermenting a hydrolysate obtained by treating biomass chemically or biologically.
The biomass that can be used includes edible and non-edible starches, grass, wood, seaweeds, organic waste resources (waste wood, agricultural and forestry byproducts, food byproducts, etc.), and so forth. Because the sugars contained in the biomass exist in the form of cellulose and hemicellulose, which are polysaccharides, they should be degraded into monosaccharides fermentable by microorganisms, such as hexoses or pentoses like glucose, xylose, arabinose, etc. For this, pretreatment and saccharification of the biomass are required.
The pretreatment is a process wherein the biomass is shredded into small pieces and treated with an acid or an alkali such that the structure in which cellulose, hemicellulose and lignin are strongly held together is disentangled and the cellulose and the hemicellulose are released at the molecular level.
The saccharification is a process wherein the cellulose and hemicellulose released at the molecular level in the pretreatment process, i.e., the polysaccharide structure, is saccharified (or hydrolyzed) using a high-concentration acid or a low-concentration acid and an enzyme to prepare a hydrolysate of monosaccharides.
When an alkali such as caustic soda, ammonia, etc. is used in the pretreatment process, the lignin is dissolved and the cellulose and hemicellulose are obtained in the form of solid cakes. After separating the sugar cakes, the monosaccharide components are obtained by conducting saccharification using an enzyme or an acid. However, the pretreatment method using an alkali has the problems in that the process is generally operated in batch mode under high-temperature and high-pressure conditions, a large amount of alkali liquid waste is generated and the solid cake causes troubles in the apparatus during a continuous process.
When sulfuric acid is used during the pretreatment and saccharification of biomass, a neutralization process is necessary for the following step. For example, when dilute sulfuric acid is used for the pretreatment and then the saccharification is performed using an enzyme, a neutralization process of adjusting the pH of the pretreated solution to a pH suitable for an enzymatic reaction is necessary. When the pretreatment is conducted using concentrated sulfuric acid and then the saccharification is performed using concentrated sulfuric acid, a neutralization process of separating (or removing) the sulfuric acid is necessary for fermentation of the saccharified hydrolysate by microorganisms in the following step. That is to say, the acid should be removed or the acid-sugar mixture should be neutralized for the following enzymatic saccharification or fermentation by microorganisms.
In order to remove sulfuric acid from a sugar-acid mixture prepared using sulfuric acid, a method of neutralizing with an alkali such as calcium hydroxide (Martinez et al., 2001) is mainly used. During this process, a large amount of gypsum is produced as waste and a significant amount of sugars are lost while separating the gypsum from the sugar solution. As another method of separating sugars and acids from the sugar-acid mixture obtained by pretreating with concentrated sulfuric acid and then performing saccharification, a continuous adsorptive separation method of using a simulated moving bed (SMB) is known. In this method, a number of chromatography (adsorption) columns selectively adsorbing and separating sugars or acids are installed and a valve and a pump are connected to each column to enable continuous sample injection and continuous product discharge, thereby continuously separating sugars and acids from the hydrolysate. However, it has problems in that the expensive chromatography (adsorption) column is used, operation is very complicated due to the complexity of apparatus configuration and, particularly, it is difficult to recycle the acid because the acid solution is diluted to about several % using water as a mobile phase fluid in chromatography column. In addition, because the adsorbent is swollen severely when it adsorbs the acid, the durability of the adsorbent worsens as the adsorption-desorption cycle is repeated.
In addition, a method of recovering the acid used in the pretreatment through an electrodialysis method has been reported recently. In the electrodialysis method, the separation is achieved based on the electrophoresis of ionic materials in the solution and the selective permeation of cations and anions through the ion-exchange membrane. A number of cation-exchange membranes and anion-exchange membranes are arranged alternatingly and the ionic materials in the solution can be separated by flowing direct current. Jaewon Lee et al. of Chungnam National University (Korea) recovered oxalic acid used for pretreatment from the hydrolysate using the electrodialysis method and Snyder et al. of Argonne National Laboratory (US) recovered sulfuric acid from the hydrolysate by filling an ionic resin in the diluate compartment of the electrodialysis apparatus. Although the acid used in the pretreatment could be separated from the hydrolysate using the electrodialysis method, the method is not applicable to high-concentration sulfuric acid.
In addition, the method based on electrodialysis is disadvantageous in that the system in which the anion- and cation-exchange membranes are arranged alternatingly is complicated and high-voltage electricity is required because the system becomes nonconducting as the ions are removed.
Therefore, in order to put the pretreatment of biomass using concentrated sulfuric acid or dilute sulfuric acid and saccharification into practical use, an economical method allowing effective recovery and recycling of sulfuric acid from the sugar-acid mixture without loss of sugars is necessary.